Temperature-dependent Raman scattering of DyScO3 and GdScO3 single crystals

We report a temperature-dependent Raman scattering investigation of DyScO3 and GdScO3 single crystals from room temperature up to 1200 {\deg}C. With increasing temperature, all modes decrease monotonously in wavenumber without anomaly, which attests the absence of a structural phase transition. The high temperature spectral signature and extrapolation of band positions to higher temperatures suggest a decreasing orthorhombic distortion towards the ideal cubic structure. Our study indicates that this orthorhombic-to-cubic phase transition is close to or higher than the melting point of both rare-earth scandates (\approx 2100 {\deg}C), which might exclude the possibility of the experimental observation of such a phase transition before melting. The temperature-dependent shift of Raman phonons is also discussed in the context of thermal expansion

Self-consistent solution for the polarized vacuum in a no-photon QED model

We study the Bogoliubov-Dirac-Fock model introduced by Chaix and Iracane ({\it J. Phys. B.}, 22, 3791--3814, 1989) which is a mean-field theory deduced from no-photon QED. The associated functional is bounded from below. In the presence of an external field, a minimizer, if it exists, is interpreted as the polarized vacuum and it solves a self-consistent equation. In a recent paper math-ph/0403005, we proved the convergence of the iterative fixed-point scheme naturally associated with this equation to a global minimizer of the BDF functional, under some restrictive conditions on the external potential, the ultraviolet cut-off $\Lambda$ and the bare fine structure constant $\alpha$. In the present work, we improve this result by showing the existence of the minimizer by a variational method, for any cut-off $\Lambda$ and without any constraint on the external field. We also study the behaviour of the minimizer as $\Lambda$ goes to infinity and show that the theory is "nullified" in that limit, as predicted first by Landau: the vacuum totally kills the external potential. Therefore the limit case of an infinite cut-off makes no sense both from a physical and mathematical point of view. Finally, we perform a charge and density renormalization scheme applying simultaneously to all orders of the fine structure constant $\alpha$, on a simplified model where the exchange term is neglected.Comment: Final version, to appear in J. Phys. A: Math. Ge

A new approach to the modelling of local defects in crystals: the reduced Hartree-Fock case

This article is concerned with the derivation and the mathematical study of a new mean-field model for the description of interacting electrons in crystals with local defects. We work with a reduced Hartree-Fock model, obtained from the usual Hartree-Fock model by neglecting the exchange term. First, we recall the definition of the self-consistent Fermi sea of the perfect crystal, which is obtained as a minimizer of some periodic problem, as was shown by Catto, Le Bris and Lions. We also prove some of its properties which were not mentioned before. Then, we define and study in details a nonlinear model for the electrons of the crystal in the presence of a defect. We use formal analogies between the Fermi sea of a perturbed crystal and the Dirac sea in Quantum Electrodynamics in the presence of an external electrostatic field. The latter was recently studied by Hainzl, Lewin, S\'er\'e and Solovej, based on ideas from Chaix and Iracane. This enables us to define the ground state of the self-consistent Fermi sea in the presence of a defect. We end the paper by proving that our model is in fact the thermodynamic limit of the so-called supercell model, widely used in numerical simulations.Comment: Final version, to appear in Comm. Math. Phy

Gene flow estimation with microsatellites in a Malagasy seed orchard of Eucalyptus grandis

International audienceEucalyptus grandis has a mixed-mating repro- ductive system. Malagasy Eucalyptus seed orchards were established 15 years ago with two aims both based on panmixia: open-pollinated seed production and genetic improvement. The panmixia hypothesis has never been confirmed in the seed orchard. From a seedling seed- orchard stand comprising 349 trees and using data obtained with six selected microsatellite markers, pater- nity analysis was performed for 724 offspring collected on 30 adult trees. Paternity assignment, based on exclu- sion procedures and likelihood-ratio method, was achieved with high accuracy; the exclusion probability value was 0.997. The outcrossing rate was very high (96.7%). More than 50% of potential male trees (199 out of 349) in the seed orchard contributed to pollination for 440 offspring of 30 progenies (8.6% of the basic population). The pollination rate from outside the seed orchard was high (39.2%), but might be due to the small size of this seed orchard. This study showed that "panmixia-like pollination" can be assume

Variation de la densité et la couleur du bois de Pterocarpus erinaceus (Poir) en fonction des conditions environnementales en Afrique de l’Ouest

National audiencePterocarpus erinaceus (Poir.) is currently spontaneous species of Guinea-Sudanese and Sudano-Sahelian areas overexploited and threatened in West Africa. This paper analyzes the density and color of the wood of the species in the agro-ecological zones of three countries in West Africa (Togo, Burkina Faso and Niger). The results of 452 samples collected from 95 trees from the three countries show significant differences in values of basic density and color parameters (P <0.05) according to agro-ecological zones (Sudan, Guinea and Sahel). At scale intra-tree, even if there is an increase in density with the number of rings counted from the pith, this increase is very small (R²≤ 0.18). This reflects the fact that age has little influence on the trees density. In addition, a significant correlation was observed between density and color parameters.Pterocarpus erinaceus (Poir.) est actuellement l’espèce spontanée des zones guinéo-soudaniennes et soudano-sahéliennes très exploitée et menacée en Afrique de l’Ouest. Le présent travail analyse la densité et la couleur du bois de l’espèce dans les zones agroécologiques de trois pays de l’Afrique de l’Ouest (Togo, Burkina Faso et Niger). Les résultats obtenus sur 452 éprouvettes prélevées de 95 arbres issus des 3 pays indiquent des différences significatives des valeurs de l’infradensité et des paramètres de couleur (P<0,05) en fonction des zones agro-écologiques (soudanienne, guinéenne et sahélienne). A l’échelle intra-arbre, même s’il ya une augmentation de la densité en fonction du nombre de cernes comptés, depuis la moelle, cette augmentation est très faible (R²≤ 0,18). Ce qui traduit le fait que l’âge des arbres influence peu l’infradensité. Par ailleurs une corrélation significative a été observée entre la densité et les paramètres de couleur

Existence of global-in-time solutions to a generalized Dirac-Fock type evolution equation

We consider a generalized Dirac-Fock type evolution equation deduced from no-photon Quantum Electrodynamics, which describes the self-consistent time-evolution of relativistic electrons, the observable ones as well as those filling up the Dirac sea. This equation has been originally introduced by Dirac in 1934 in a simplified form. Since we work in a Hartree-Fock type approximation, the elements describing the physical state of the electrons are infinite rank projectors. Using the Bogoliubov-Dirac-Fock formalism, introduced by Chaix-Iracane ({\it J. Phys. B.}, 22, 3791--3814, 1989), and recently established by Hainzl-Lewin-Sere, we prove the existence of global-in-time solutions of the considered evolution equation.Comment: 12 pages; more explanations added, some final (minor) corrections include

Phase transition close to room temperature in BiFeO3 thin films

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, displays a reversible temperature-induced phase transition at about 100\circ, thus close to room temperature.Comment: accepted in J. Phys.: Condens. Matter (Fast Track Communication

Non-perturbative embedding of local defects in crystalline materials

We present a new variational model for computing the electronic first-order density matrix of a crystalline material in presence of a local defect. A natural way to obtain variational discretizations of this model is to expand the difference Q between the density matrix of the defective crystal and the density matrix of the perfect crystal, in a basis of precomputed maximally localized Wannier functions of the reference perfect crystal. This approach can be used within any semi-empirical or Density Functional Theory framework.Comment: 13 pages, 4 figure

Longitudinal cohort of HIV-negative transgender women of colour in New York City: protocol for the TURNNT ('Trying to Understand Relationships, Networks and Neighbourhoods among Transgender women of colour') study.

IntroductionIn the USA, transgender women are among the most vulnerable to HIV. In particular, transgender women of colour face high rates of infection and low uptake of important HIV prevention tools, including pre-exposure prophylaxis (PrEP). This paper describes the design, sampling methods, data collection and analyses of the TURNNT ('Trying to Understand Relationships, Networks and Neighbourhoods among Transgender women of colour') study. In collaboration with communities of transgender women of colour, TURNNT aims to explore the complex social and environmental (ie, neighbourhood) structures that affect HIV prevention and other aspects of health in order to identify avenues for intervention.Methods and analysesTURNNT is a prospective cohort study, which will recruit 300 transgender women of colour (150 Black/African American, 100 Latina and 50 Asian/Pacific Islander participants) in New York City. There will be three waves of data collection separated by 6 months. At each wave, participants will provide information on their relationships, social and sexual networks, and neighbourhoods. Global position system technology will be used to generate individual daily path areas in order to estimate neighbourhood-level exposures. Multivariate analyses will be conducted to assess cross-sectional and longitudinal, independent and synergistic associations of personal relationships (notably individual social capital), social and sexual networks, and neighbourhood factors (notably neighbourhood-level social cohesion) with PrEP uptake and discontinuation.Ethics and disseminationThe TURNNT protocol was approved by the Columbia University Institutional Review Board (reference no. AAAS8164). This study will provide novel insights into the relationship, network and neighbourhood factors that influence HIV prevention behaviours among transgender women of colour and facilitate exploration of this population's health and well-being more broadly. Through community-based dissemination events and consultation with policy makers, this foundational work will be used to guide the development and implementation of future interventions with and for transgender women of colour

Genetic diversity and the emergence of ethnic groups in Central Asia

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